SOURCE

var arr = [1,2,3,4,,5,6];

console.log(arr.filter(item=>item<2))


Array.prototype.myFilter = function(fn){
    if(typeof fn !== 'function'){
        throw Error('must be a function')
    }

    const arr = this.slice();
    let result = [];

    for(let i=0; i< arr.length; i++){
        let currentResult = fn(arr[i]);
        if(currentResult){
            result.push(arr[i])
        }
    }

    return result;

}

console.log(arr.myFilter(item=>item<2))